The metal fluoride crystal is a metal compound crystal that is expected for various applications. For example, a single crystal such as a calcium fluoride and a magnesium fluoride has a high permeability, a low refractive index, and a low dispersion over a wide range of wavelength band from vacuum ultraviolet to infrared, and has excellent chemical stability. Therefore, such a single crystal is used for a window material, a lens, a prism, and the like, as an optical material in a wide range of areas. In particular, the single crystal is expected as a window material, a light source system lens, an illumination system lens, and a projection system lens of an apparatus such as an ArF laser (193 nm) and a stepper that uses an F2 laser (157 nm) light source (reduction projection exposure apparatus) developed as a next-generation short wavelength light source in an optical lithography technique.
A lithium calcium aluminum fluoride (hereinafter, referred to also as LiCAF) single crystal is a crystal that is widely used as a laser oscillation element, and a laser oscillation element to which a dopant such as a cerium (Ce) element is added has been known (Patent Document 1).
On the other hand, the metal fluoride crystal has been developed as a neutron scintillator used for a neutron detector. For example, a neutron scintillator in which a europium (Eu) element is added to a LiCAF single crystal has been known (Patent Document 2). The neutron scintillator is a substance that absorbs a neutron to emit fluorescence when the neutron collides with the neutron scintillator.
Various types of performance of a neutron detector that uses a scintillator, the detection efficiency and count rate for a neutron, and the discrimination ability for a neutron and a γ-ray (hereinafter, referred to also as n/γ discrimination ability) depend on a substance constituting the scintillator. The light emission intensity of the scintillator affects the detection efficiency. The speed of attenuation of fluorescence affects the count rate. It should be noted that the detection efficiency represents a ratio of the number of radiation counted by a detector to the number of radiation that is emitted from a radiation source and enters the detector, and the count rate represents the number of radiation counted in a unit time.